1. Field of the Technology
This invention relates generally to magnetic read heads having read sensors for reading information signals from a magnetic medium, and more particularly to an improved seed layer structure for hard bias layers formed adjacent a read sensor of the current-perpendicular-to-the-planes (CPP) type.
2. Description of the Related Art
Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks are commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive read (MR) sensors, commonly referred to as MR heads, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an “MR element”) as a function of the strength and direction of the magnetic flux being sensed by the MR layer. A common type of MR sensor is the giant magnetoresistance (GMR) sensor which manifests the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers. GMR sensors using only two layers of ferromagnetic material (e.g., nickel-iron (NiFe), cobalt (Co), or nickel-iron-cobalt (NiFeCo)) separated by a layer of nonmagnetic material (e.g., copper (Cu)) are generally referred to as spin valve (SV) sensors manifesting the SV effect. In an SV sensor, one of the ferromagnetic layers, referred to as the pinned layer, has its magnetization typically pinned by exchange coupling with an antiferromagnetic (e.g., nickel-oxide (NiO), iridium-manganese (IrMn) or platinum-manganese (PtMn)) layer.
The magnetization of the other ferromagnetic layer, referred to as the free layer, however, is not fixed and is free to rotate in response to the field from the information recorded on the magnetic medium (the signal field). In the SV sensors, SV resistance varies as the cosine of the angle between the magnetization of the pinned layer and the magnetization of the free layer. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in direction of magnetization in the free layer, which in turn causes a change in resistance of the SV sensor and a corresponding change in the sensed current or voltage. In addition to the magnetoresistive material, the GMR sensor has conductive lead structures for connecting the GMR sensor to a sensing means and a sense current source. Typically, a constant current is sent through the GMR sensor through these leads and the voltage variations caused by the changing resistance are measured via these leads.
To illustrate, FIG. 1 shows a prior art SV sensor 100 (“read sensor 100”) of the current-perpendicular-to-the-planes (CPP) type having side regions 104 and 106 separated by a central region 102. A free layer 110 is separated from a pinned layer 120 by a non-magnetic, electrically-conducting or insulating spacer 115. Spacer 115 may be made of electrically-conductive materials if sensor 100 is a GMR sensor, or alternatively, electrically-insulative materials if sensor 100 is a tunnel magnetoresistive (TMR) sensor. The magnetization of pinned layer 120 is fixed by an AFM pinning layer 121, which is formed on a shield layer 123 which may reside on a substrate (not shown in FIG. 1). Cap layer 108, free layer 110, spacer layer 115, pinned layer 120, and AFM pinning layer 121 are all formed in central region 102. Read sensor layers of read sensor 100 are generally sandwiched between shield layers 123 and 125, which together serve as a shield and as leads for the sensor.
Conventionally, hard bias layers 130 and 135 are formed in side regions 104 and 106 in order to stabilize free layer 110. These hard bias layers 130 and 135 are typically formed of a cobalt-based alloy which is sufficiently magnetized and perhaps shielded so that the magnetic fields of the media and/or the write head do not effect the magnetism of the hard magnets. Seed layers 150 and 155 are also deposited in side regions 104 and 106 underneath hard bias layers 130 and 135 to set a texture for the successful deposition of the hard magnets by promoting a desired c-axis in plane orientation. To perform effectively, hard bias layers 130 and 135 should have a high coercivity, a high MrT (magnetic remanence×thickness), and a high in-plane squareness on the magnetization curve. A preferred cobalt-based alloy for hard bias layers 130 and 135 is cobalt-platinum (CoPt) or cobalt-platinum-chromium (CoPtCr), while seed layers 150 and 155 typically comprise chromium (Cr) or other suitable metallic element.
Thus, as illustrated in FIG. 1, seed layers 150 and 155 and hard bias layers 130 and 135 are formed in side regions 104 and 106, respectively, and provide longitudinal bias for free layer 110. Cap layers 140 and 145 are formed over these hard bias layers 130 and 135, respectively, in the side regions 104 and 106. Seed layers 150 and 155 are formed over insulator layers 190 and 192, respectively, which are in turn formed directly over shield layer 123. Shield layers 123 and 125, which are “leads” of the sensor 100, provide electrical connections for the flow of the sensing current Is from a current source 160 to the sensor 100. In read sensors of the CPP type, sensing current Is is generally forced through the layers in central region 102 but not through side regions 104 and 106. Sensing means 170, which is connected to these leads, senses the change in the resistance due to changes induced in the free layer 110 by the external magnetic field (e.g. field generated by a data bit stored on a disk). One material for constructing these leads/shield layers 140 and 145 is a highly conductive material, such as a metal.
FIG. 2 shows a prior art read sensor 200 of the CPP type, similar to prior art read sensor 100 (FIG. 1), having side regions 204 and 206 separated by a central region 202. A free layer 210 is separated from a pinned layer 220 by a non-magnetic, electrically-conducting or insulating spacer 215. The magnetization of pinned layer 220 is fixed by an AFM pinning layer 221, which is formed on a shield layer 223 which may reside on a substrate (not shown in FIG. 2). Cap layer 208, free layer 210, spacer layer 215 and pinned layer 220 are all formed in central region 202. Unlike prior art read sensor 100 of FIG. 1, prior art read sensor 200 of FIG. 2 is a partial mill design with materials of AFM pinning layer 221 of sensor 200 extending into side regions 204 and 206. By “partial mill design”, it is meant that the read sensor layers are not fully etched or milled in side regions 204 and 206 prior to the deposition of the seed, hard bias, and lead materials. A partial mill design may be desirable in order to better align free layer 210 with hard bias layers 230 and 235.
As illustrated in FIG. 2, seed layers 250 and 255 and hard bias layers 230 and 235 are formed in side regions 204 and 206, respectively. Hard bias layers 230 and 235 provide longitudinal biasing for free layer 210. Cap layers 240 and 245 are formed over these hard bias layers 230 and 235, respectively, in side regions 204 and 206. Seed layers 250 and 255 are formed over insulator layers 290 and 292, respectively, which are in turn formed directly over AFM pinning layer 221.
Similarly, as described earlier in FIG. 1, shield layers 223 and 225 which serve as “leads” of the sensor 200 provide electrical connections for the flow of the sensing current Is from a current source 260 to the sensor 200. Sensing current Is is generally forced through the layers in central region 202 but not through side regions 204 and 206. Sensing means 270, which is connected to these leads, senses the change in the resistance due to changes induced in the free layer 210 by the external magnetic field (e.g. field generated by a data bit stored on a disk).
Again, to perform effectively, hard bias layers of a CPP read sensor should have a high coercivity, a high MrT, and a high in-plane squareness on the magnetization curve. What are needed are methods and apparatus for improving hard magnet properties in read sensors of the CPP type.